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When I push the startup button and my iMac starts to load about 2/3 the way through it will abruptly turn back off. Is there a sequence of keys that can be pushed while it’s trying to turn back on to keep it from turning off? I do not have the startup disk.
iMac 21.5", macOS 10.14
1. Start up from macOS Recovery
To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… https://support.apple.com/en-us/HT204904#notupdated
Command (⌘)-R
Install the latest macOS that was installed on your Mac.
Option-⌘-R
Upgrade to the latest macOS compatible with your Mac.
Shift-Option-⌘-R
Install the macOS that came with your Mac, or the closest version still available.
2. Decide whether to erase (format) your disk
If you need to erase your disk before installing macOS, select Disk Utility from the Utilities window, then click Continue. You probably don't need to erase, unless you're selling or giving away your Mac or have an issue that requires you to erase. Learn more about when and how to erase.
3. Install macOS
After starting up from macOS Recovery, follow these steps to install macOS:
1 Choose Reinstall macOS (or Reinstall OS X) from the Utilities window.
2 Click Continue, then follow the onscreen instructions to choose your disk and begin installation.
3 If the installer asks to unlock your disk, enter the password you use to log in to your Mac. If it doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk.
1 Please allow installation to complete without putting your Mac to sleep or closing its lid. During installation, your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.
If your Mac restarts to a setup assistant, but you're selling or giving it away, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.

If you never installed macOS Sierra 10.12.4
If macOS Sierra 10.12.4 or later was never installed on your Mac, macOS Recovery works differently:
• Command-R is still the recommended way to start up from macOS Recovery. This combination makes sure that the installation isn't associated with your Apple ID, which is important if you're selling or giving away your Mac.
• Option-Command-R installs the macOS that came with your Mac, or the closest version still available.
• Shift-Option-Command-R isn't available.
I have tried all the combinations that are suggested above with no success. I press the power button and than you hear the Mac starting sound. I immediately press the different combinations, one combination at a time, until the loading bar gets to be about 3/4 of the way than it abruptly turns off. Is there something that I’m missing or is there anything else that I can try?
No, but see if this helps...
Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?
Unplug the power cable for about 10 seconds, plug back in and try again. If that allows you to boot up fully but still shuts down later then you have a failing or failed power supply.
Unfortunately holding the shift key down during startup did not solve this problem. Do you have any other suggestions that I could try?
Are you using a Wireless KB perchance?
If so try a Wired KB, even a cheap PC USB KB.
startup issue on iMac
